Detection of faults in electrical machines is an important issue for ensuring a reliable functioning of the machine. The earlier a fault is detected the better the maintenance can be planned thus avoiding uncontrolled machine stops and serious damages from occurring. Conventionally both mechanical and electrical characteristics of machines are used for identifying faults. For example, mechanical vibrations and the behavior of electrical currents within the machine have been used as fault indicators. Circulating currents between parallel connected winding branches have been found to be a good fault indicator since they are ideally equal to zero for a normal condition and increase due to any asymmetry within the machine. Circulating currents enable detection of faulty conditions at a very early stage, for example a crack of the rotor bars (before the bar is totally broken), inter-turn short-circuits, eccentricity problem, etc.
GB2157005A discloses a spectrum analysis of circulating currents between two parallel winding branches in a stator of a two-pole alternator for detecting different types of faults. According to GB2157005A the relevant harmonics for analyzing the spectrum of the circulating currents are even harmonics of the supply frequency (50 Hz).
WAN SHUTING et al: “A Compositive Diagnosis Method on Turbine-Generator Rotor Winding Inter-turn Short Circuit Fault” discloses detecting an inter-turn short circuit in a rotor by analysing circulating currents.
US7253634B1 discloses detecting stator ground faults in a generator by analysing circulating currents. US7253634B1 discloses inter alia measuring the circulating currents in different electrical phases of the machine.
MOHAMED EL HACHEMI BENBOUZID: “A Review of Induction Motors Signature Analysis as a Medium for Faults Detection” discloses a fault detection method based on signature analysis in induction motors.
Though theoretically circulating currents provide an improved method of detecting machine faults at an early stage, there are a number of practical difficulties in realizing a method of acquiring and processing the relevant data. Such practical difficulties include finding the best fault indicators and learning to interpret the results correctly. A circulating current can be measured at many different locations of the machine, and arranging the measurement is difficult because the current sensors may need to be integrated into the windings of the machine. Therefore, this method is most appropriate for high power motors and generators, where there is sufficient room to locate the current sensors inside the machine. Furthermore, special skill is needed for interpreting the measurement results correctly in order to actually be able to detect a fault condition. There remains a desire to improve the existing fault detection methods in order to more reliably detect different types of faults.